The caster summons a scroll of fine parchment from thin air. Nothing that is written in the scroll cannot be seen  nor summoned by others without your consent.  When a scroll is filled up, a new one is summoned though the old one can still be called forth. You can access another creatures scrolls by using  a spell slot of 5th level or higher but doing so requires a successful contested INT check or using a scrying spell of 6th level or higher. Another way to access them is to visit the Great Archives in the Sanctum of All, a demiplane constantly shifting in position. The scrolls of every caster their is are kept there and accessing them dose not require consent or contest, though there are an extreme amount of mages, scholars and warforged who reside in the plane.